Solid-state memory devices are playing very important roles in the modern information-based society. They exist in most of electronic products used in our daily life, including personal computers, mobile phones, cameras, audio players, automotive systems and global positioning systems, etc. Depending on whether or not data can be retained after powered off, memory devices may be mainly divided into two types: nonvolatile memory and volatile memory. The volatile memory is not capable of retaining data after powered off, while the nonvolatile memory can retain data even though the power is turned off.
Conventional memories are mainly based on magnetic memories and optical memories. Novel memory devices based on the semiconductor technology have gradually taken the dominant position in the memory market due to their great advantages. In particular, the emergence of dynamic random access memory (DRAM) and flash memory leads to a revolution in the field of memory. However, with the continuous development in the semiconductor industry, the size of the device is reduced continuously, and the DRAM as well as the flash memory will be shrunk to their physical limits. In particular, after entering a 22 nm technology node, disadvantages of these two types of devices have emerged, so that they are not able to meet requirements for the development of memory device. Various novel memory structures have been put forward, such as a ferroelectric memory, a magnetic memory, a phase change memory and a resistive-switching memory, etc., in which the resistive-switching memory has become a hot topic due to the advantages such as low cost, high speed, and low voltage, etc.
The resistive-switching memory (RRAM) is a brand-new nonvolatile memory, and its resistance value may be switched reversibly between a high resistance status and a low resistance status by applying an external electric field, so that the information storage is achieved. In the RRAM, the resistive-switching material may present two resistance states completely different form each other (low resistance state and high resistance state) under the same read voltage, and both of the resistance states can be retained for a long time after the erase voltage is removed, so that the nonvolatile storage of data may be achieved. The RRAM comprises a plate capacitor structure in which a layer of the resistive-switching material is interposed between two metal electrodes. The RRAM has become the most promising competitor for the next-generation memory due to its simple structure and excellent performance, and therefore it is paid close attention and studied widely.
Currently, researches on the RRAM are mainly focused on the selection and preparation of the resistive-switching material and the metal electrode. As for the resistive-switching material, resistive-switching characteristics of transition metal oxides, perovskite oxides, rare metal oxides and ferromagnetic materials have been studied. As for the electrode, influences of various electrode metal materials on resistive-switching characteristics of RRAM have been studied. Great progress has been made regarding the RRAM device. The RRAM is turned on mainly depending on the movement of oxygen vacancies inside the resistive-switching material or the movement of metal ions in the gate by which a conductive path is formed so that the resistive-switching material is switched from a high resistance state to a low resistance state.
A metal-resistive-switching-material-metal (MIM) plate capacitor structure is adopted in conventional RRAM structures, as shown in FIG. 1. The structure mainly consists of a top electrode 1, a bottom electrode 3 and a resistive-switching material 2 interposed between the top and bottom electrodes. The operation states are as follows: the resistive-switching material exhibits a high resistance state in an initial state; when the voltage between the two electrode plates is increased to a certain value, the current between the electrode plates is increased drastically and the resistive-switching material is switched into a low resistance state, and the voltage at this time is referred to as Vset; and after the applied voltage reaches a certain value, the current between the two electrode plates is decreased drastically, and the voltage at this time is referred to as Vreset. The reason that the current of the RRAM device is increased drastically is mainly due to the conductive path formed within the resistive-switching material. When the voltage is increased to Vset, oxygen vacancies or charged metal ions in the material are moved by application of the internal electric field, and thus a conductive path is formed locally between the top and bottom electrodes, so that the current is increased drastically. When the voltage becomes Vreset, the current is increased drastically due to the very low resistance at this time, so that the path is fused and the resistive-switching material switches into high resistance state, and then the current is decreased drastically. The top and bottom electrodes of the conventional RRAM adopt a plate structure, and the electric field is distributed uniformly between the two electrode plates. Since the electrodes may be uneven locally, the electric field of relatively high intensity is locally formed to lead a movement of ions, so that a conductive path is formed. However, the position where such conductive path is formed is random, so it is disadvantageous to the stability of the RRAM performance. Moreover, the voltage Vset is relatively high, which is disadvantageous to reducing the power of the device. Therefore, it is necessary to locate the position of the electric field intensity through an appropriate structure.